Re: Blank page or 500 Internal Server error
The problems are caused by your code failing. A blank page is usually a PHP error so checking your PHP error_log within your Plesk panel will provide causes as to what the problem is. Usually the best course of action is to re-upload your website or restore from a recent backup.
A 500 internal server error is usually related to a scripting issue using CGI or Perl scripts. Again, this means the script is unable to execute and this generic error which does not give much away means you need to check your scripts or simply upload / restore to your last working copy.
Both of these errors means a quick call to your web designer or just restore from your latest backup in the Backup manager (You do take backups right?!)